The present invention relates to a process for the preparation of isocyanates by phosgenation of the corresponding amines in the gas phase using microstructure mixers for rapid mixing of the educts.
To carry out a chemical reaction in a continuous procedure, the reactants must be fed continuously to a chemical reactor and brought into intimate contact (i.e., mixed thoroughly) with the aid of a mixing element (mixer). As a rule, several reactions, so-called main and side reactions, proceed in the reactor when the reactants come into contact. The aim of the process engineer is to conduct the mixing and the reactions in a manner such that the highest possible yield of the desired product is achieved.
The quality of the mixing and the influence of the mixing element on the yield of the desired product depend on the ratio of the rate of the chemical reaction (determined by the reaction kinetics) to the rate of mixing. If the chemical reactions are slow reactions, as a rule the chemical reaction will be substantially slower than the mixing. The overall rate of reaction and the yield of desired product is then determined by the slowest step of the chemical reaction and by the mixing properties (residence time distribution, macromixing) of the chemical reactor used. If the rates of the chemical reactions and the rate of mixing are of the same order of magnitude, complex interactions between the kinetics of the reactions and the local mixing properties (determined by the turbulence in the reactor used and at the mixing element (micromixing)) arise. If the rates of the chemical reactions are substantially faster than the rate of mixing, the overall rates of the reactions and the yields obtained are substantially determined by the mixing (i.e., by the local time-dependent speed and concentration field of the reactants, the turbulence structure in the reactor and at the mixing element). (Brodkey, Turbulence in Mixing Operations, Academic Press, 1975).
According to the prior art, a number of mixing elements have been employed to carry out fast reactions in a continuous procedure. A distinction may be made here in principle between dynamic mixers (e.g., stirrers, turbines and rotor-stator systems), static mixers (e.g., Kenics mixers, Schaschlik mixers and SMV mixers), and jet mixers (e.g., nozzle mixers or T mixers). See, e.g., Chem. Ing. Tech. MS 1708/88; Fortschr. Verf. Technik 23,1985,373; and Ind. Eng. Chem. Res. 26,1987,1184.
For rapid mixing of starting substances in rapid reactions with the potential for undesirable secondary or side reactions, nozzle mixers are preferably employed. This particularly applies to reactions which proceed in the gas phase.
It has been known for a long time that isocyanates could be produced by reacting amines in the gas phase. However, gas phase reaction has acquired industrial importance only since the development of a process in which the problems of partial decomposition of polyfunctional amines during evaporation and of the tendency towards the formation of polymers during the phosgenation are eliminated (EP-A-289,840).
EP-A 0,289,840; 0,676,392; and 0,749,958 describe processes for the preparation of aliphatic di- and triisocyanates from the corresponding di- and triamines respectively by phosgenation in the gas phase. In these processes, the educt gas streams are passed into a tubular reactor for reaction. Mixing of the reactants takes place on entry into the tubular space through nozzles or a combination of a nozzle and an annular gap between the nozzle and tube. A Reynolds number of Re.gtoreq.4,700 in the tube is taught to be an essential criterion for the mixing.
In EP-A 0,570,799, a jet mixer is used to mix the educts in the preparation of aromatic diisocyanates by phosgenation in the gas phase. EP-A 0,699,657 describes a multiple nozzle injection system for mixing the educts.
In the jet or nozzle mixers described in these disclosures, one of the two starting components is atomized into the other component(s) at a high flow rate. The kinetic energy of the sprayed stream is substantially dissipated behind the nozzle, i.e., it is converted into heat by turbulent breakdown of the stream into eddies and further turbulent breakdown of the eddies into ever smaller eddies. The eddies contain the particular starting components which are present side-by-side in the fluid balls (macromixing). A small degree of mixing by diffusion occurs at the edges of these initially larger structures at the start of the turbulent breakdown of the eddies. However, complete mixing is achieved only when the breakdown of the eddies has progressed to the extent that, when eddy sizes of the order of magnitude of the concentration microdimension (Batchelor length) (J. Fluid Mech. 5, 1959, 113; Chem. Eng. Sci. 43, 1988, 107) are reached, the diffusion is rapid enough for the starting components to be mixed completely with one another in the eddies. The mixing time required for complete mixing depends substantially on the specific energy dissipation rate, the characteristics of the specific materials and the geometry of the apparatus.
When nozzle or jet mixers are used in accordance with the prior art, the time for breakdown of the eddies elapses before complete mixing by diffusion. For very fast reactions, this means that either very high energy dissipation rates must be established in order to avoid undesirable side and secondary reactions, or, in the case of reactions with even higher rates of reaction, the corresponding reactions are not carried out to the optimum (i.e., only by-products or secondary products are formed).